How do cells maintain structure, connections & organize activities?

How do cells maintain structure, connections &

organize activities?

Proteins!

• Ultimately responsible for each of these activities.

• Proteins provide structure, allow movement & mediate interactions

Some plasma membrane proteins

Transporta) Passiveb) Active

Enzymes

Initiating enzyme cascades

Intercellular junctions

Cell-cell recognition

Attach to cytoskeleton;Motor proteins

Tight junctions staple neighboring cells exposed to chemical stress

Gap junctions allow rapid communication & sharing between neighbors

Desmosomes bind neighboring cells exposed to mechanical stress

Extracellular environment • Space between cells• Extracellular matrix: sticky fluid

derived from plasma– nutrients for cells - glycoprotein, salts,

amino acids, etc.

– Cellular wastes – CO2, lactic acid, etc.

• Cells must exchange nutrients & wastes with the environment.

Cell membranes areselectively permeable

• Some compounds pass uninhibited through membrane (passive diffusion), some require assistance from membrane proteins (facilitated diffusion), and some require assistance AND energy expenditure (active transport)

1. Diffusion– Passive diffusion– Carrier or channel-mediated (facilitated) diffusion

2. Active Transport– Pumps, bulk transport

PM + proteins mediate transport

Passive (Diffusion & Osmosis) or Active

Simple diffusion; P

Osmosis; P

Channel-mediated; P

Carrier- mediated; A

What determines whether transport is passive or

active?

What determines rate of transport?

First, terminology• Solvent: The predominant liquid or

gas in a solution• Solute: The stuff that is dissolved in a

solution• Diffusion: The net movement of

solute from a higher to a lower concentration (Concentration gradient), until equilibrium is achieved. Uses intrinsic Kinetic Energy (KE).

Passive diffusion

• Kinetic energy causes particles to move

• Diffusion occurs due to random collisions between these energized particles

Osmosis + Diffusion• Both are happening all the time

across cell membranes

• Osmosis (H20) occurs RAPIDLY, diffusion (solutes) occurs SLOWLY

• H20 moves into cells with high solute concentration and out of cells with low solute concentration

Cytoskeleton• Cytoskeleton = cell

skeleton• All cells contain

structural filaments:– Microfilaments– Intermediate

filaments– Microtubules– Thick filaments

(muscle cells)

• Made of proteins

Microfilaments• Actin strands• Primarily in periphery of

cell• Functions:

– Anchor cytoskeleton to integral proteins of cell membrane

– Interact with myosin to promote cell shortening (Ex: muscle cells)

Microvilli• Microfilaments

(actin)• Increase SA of cell

– maximizes absorptive surface (Ex: intestinal walls)

• No movement

Intermediate filaments• (7-11nm)• Most durable

cytoskeletal fiber• Located throughout

cell; High # in superficial layers of skin

• Functions– Provides shape to cell– Stabilize (encase)

organelles

Thick filaments

• Only found in muscle cells, interact with actin to form a contraction

Microtubules• tubulin protein subunits;

ALL cells contain these

• Microtubular array centered near the nucleus (@ centrosome)

• Functions– Cell shape & rigidity– Anchor organelles; RR tracks

for organelle movement– Forms spindle apparatus– Forms centrioles, basal

bodies, parts of flagella

Centrioles & Basal bodies• Centrioles

– Form anchors of spindle apparatus

– Anchor is independent of spindle apparatus

• Basal bodies– Anchors flagella & cilia

to a cell– Anchor is an extension

of flagella & cilia

Cilia & Flagella• Cilia

– Rhythmic beating (rowing team) moves fluids & particles across cell surface

• Where might you find these?

• Flagella– Whip-like motion

moves cell• Where do you find

them?

Movement

• Dynein arms (red) anchored to microtubule

• Grab adjacent microtubule and “walk” along

• Produces bending• Show “flagella & cilia”